Recently, televisions having a large and thin screen as represented by liquid crystal displays (LCD) and plasma display panels (PDP) are the mainstream of television markets. In the televisions using liquid crystal displays, blur of images is reported to occur as compared with self-emitting plasma displays, due to a system called “hold display” of continuously displaying the same image for 16.7 microseconds ( 1/60 second). This blur is considered to occur due to a mismatch between a movement of eyeballs and a movement of an image because of a continuous still image on the screen while the eyeballs move continuously following the movement of images on the screen. To improve this moving image characteristic, shortening a holding time is effective. Therefore, nowadays many liquid crystal displays employ double-speed display.
To achieve double-speed display, there are a method of inserting a black-screen frame between original-image frames formed by existing video signals, and a method of inserting an interpolated frame between original-image frames. FIG. 14 is a schematic diagram for explaining a conventional technique. As the method of inserting an interpolated frame, there are a double-speed display method of inserting one interpolated frame between original-image frames, and a quadruple-speed display method of inserting three interpolated frames between original-image frames, as depicted in FIG. 14. As a method of inserting an interpolated frame, there is also a quintuple-speed display method of converting a 24-frame cinema video into 120 frames per second (fps). Because an interpolated frame is generated as an intermediate image to interpolate between original-image frames, blur occurs in displayed images. Therefore, when the number of inserted frames increases, clearness of moving images (or moving pictures) is lost.
Conventionally, to decrease the blur that occurs in an enlarged (or interpolated) frame, an enhancement processing method (enhancer) for enhancing an edge of an image has been disclosed. The enhancement processing is performed to detect an edge of an image for each pixel that forms a frame, and apply an enhancement filter to an edge when detected, to distinctly emphasize a brightness difference to a direction of the edge. There have been disclosed a method of controlling a coefficient of an enhancement filter used for enhancement processing in a time axis direction, based on a result of searching a motion vector, and a method of generating brightness of an interpolated frame based on reliability of a motion vector for each pixel group.
Such a conventional technology is exemplarily disclosed in Japanese Laid-open Patent Publication No. 2006-91412 and Japanese Patent Application National Publication No. 2004-518339
The conventional techniques described above have a problem that proper enhancement processing may not be performed on a stream into which interpolated frames are inserted. For example, when conventional enhancement processing is applied to a stream into which interpolated frames are inserted, original-image frames that are not required to be enhanced are also enhanced, and therefore proper enhancement is not performed. Further, the conventional enhancement processing uniformly applies an enhancement filter to whole a frame. However, because the blur of the interpolated frames does not uniformly occur in all pixels in a frame, there are problems such that a still text subtitle is enhanced too much, and thus proper enhancement cannot be made.